Is MIRI doing genuine high-quality research?
Whether or not it's produced good research is a question I can't answer.  But I find it interesting and it seems reliable.

For instance, this paper on Parametric Bounded Löb's Theorem and Robust Cooperation of Bounded Agent is drawn from Cornell University's site.  Likewise this recent paper on Logical Induction. 

That said, they also seem to publish papers that have not been subject to peer review, which is an issue not specific to the site, and seems to more widespread phenomenon per easy access to digital files in general. 

In relation to MIRI specifically, possibly this is a function of an an applied field advancing so rapidly that publication in journals is not seen as a priority, or even particularly relevant.